def prost(x):
      k = 0
      for d in range(1,x+1):
                if x%d==0:
                       k+=1
      if k == 2:
          return True
      else:
          return False
maxx = 0
num = int(input())
if num >= 1000 and num <= 9999:
    n1 = str(num)
    for cifra in range(len(n1)):
           if prost(int(n1[cifra])) == True:
               maxx = max(maxx, int(n1[cifra]))
    for chislo in range(len(n1)-1):
           if prost(int(n1[chislo]+n1[chislo+1])) == True:
               maxx = max(maxx, int(n1[chislo] +n1[chislo+1]))
    for chislo in range(len(n1)-2):
           if prost(int(n1[chislo]+n1[chislo+1]+n1[chislo+2])) == True:
               maxx = max(maxx, int(n1[chislo]+n1[chislo+1] +n1[chislo+2]))
    for chislo in range(len(n1)-3):
            if prost(int(n1[chislo]+n1[chislo+1]+n1[chislo+2]+n1[chislo+3])) == True:
                maxx = max(maxx, int(n1[chislo] +n1[chislo+1]+n1[chislo+2]+n1[chislo+3]))

    print(maxx)
else:
    print(0)
               
               
           
              
    
    
